High electron mobility transistor structure and method of making the same

ABSTRACT

A transistor includes a first layer over a substrate. The transistor also includes a second layer over the first layer. The transistor further includes a carrier channel layer at an interface of the first layer and the second layer. The transistor additionally includes a gate structure, a drain, and a source over the second layer. The transistor also includes a passivation material in the second layer between an edge of the gate structure and an edge of the drain in a top-side view. The carrier channel layer has a smaller surface area than the first layer between the edge of the gate structure and the edge of the drain in the top-side view.

PRIORITY DATA

The present application is a continuation application of U.S. patentapplication Ser. No. 14/855,460, filed Sep. 16, 2015, which is acontinuation of U.S. patent application Ser. No. 14/185,231, filed Feb.20, 2014, now U.S. Pat. No. 9,147,743, Issued Sep. 29, 2015, which is adivisional of U.S. patent application Ser. No. 13/354,768, filed Jan.20, 2012, now U.S. Pat. No. 8,680,535, Issued Mar. 25, 2014, whichclaims priority of U.S. Provisional Application No. 61/579,903, filedDec. 23, 2011, hereby incorporated by reference in their entireties.

BACKGROUND

Group-III group-V compound semiconductors (often referred to as III-Vcompound semiconductors), such as gallium nitride (GaN) and its relatedalloys, have been under intense research in recent years due to theirpromising applications in electronic and optoelectronic devices. Thelarge band gap and high electron saturation velocity of many III-Vcompound semiconductors also make them excellent candidates forapplications in high temperature, high voltage, and high-speed powerelectronics. Particular examples of potential electronic devicesemploying III-V compound semiconductors include high electron mobilitytransistor (HEMT) and other heterojunction bipolar transistors.

During operation, a HEMT forms a large surface electric field around thegate edge, which affects the depletion region curve in a drift regionbetween the gate structure and the drain. While a large electric fieldis one of the benefits of HEMT for use in power applications, the shapeand size of the depletion region during operation can negatively affectthe breakdown voltage for the device. When negative bias is applied tothe gate of the HEMT, a depletion region curve is formed directly underthe gate and causes a high surface electric field around the gate. Theelectric field concentration around the gate decreases breakdownvoltage.

In order to decrease breakdown voltage, a metallic field plate issometimes added over or next to the gate structure over a passivationlayer between the gate structure and the drain. The field platemodulates the surface electric field distribution and enhances thebreakdown voltage. However, new structures with high breakdown voltagefor III-V compound semiconductor based transistors and methods forforming them continue to be sought.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1A to 1D are cross sectional views of a high electron mobilitytransistor (HEMT) structure in accordance with various embodiments ofthe present disclosure.

FIGS. 2A to 2B are top views of a portion of HEMT structure of FIG. 1Afrom view plane 110 of FIG. 1A in accordance with various embodiments ofthe present disclosure.

FIG. 3 is a flow chart of methods for forming a HEMT structure inaccordance with certain embodiments of the present disclosure.

FIG. 4 is a flow chart of methods for forming a HEMT structure inaccordance with certain embodiments of the present disclosure.

FIGS. 5A to 5E are cross section schematics of a HEMT in various stagesof formation according to method embodiments of the present disclosure.

FIGS. 6A and 6B are plots of electric field versus distance for acomparative HEMT and a HEMT in accordance with the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

A novel structure for group-III group-V (referred to as III-Vhereinafter) semiconductor based transistors and methods for forming thestructures are provided. Throughout the description, the term “III-Vcompound semiconductor” refers to compound semiconductor materialscomprising at least one group III element and one group V element. Theterm “III-N compound semiconductor” refers to a III-V compoundsemiconductor in which the group V element is nitrogen. Example stagesof manufacturing an illustrative embodiment of the present disclosureare disclosed. Those skilled in the art will recognize that othermanufacturing steps may need to take place before or after the describedstages in order to produce a complete device. Other stages ofmanufacturing that may be substituted for some of the example stages maybe discussed. Those skilled in the art will recognize that othersubstitute stages or procedures may be used. Throughout the variousviews and illustrative embodiments of the present disclosure, likereference numbers are used to designate like elements.

The present disclosure provides a structure and a method to form III-Vcompound semiconductor-based transistors having a high breakdownvoltage. FIGS. 1A to 1D show various example power transistor devices100 a to 100 d according to various embodiments of the presentdisclosure. The power transistor device may be a high electron mobilitytransistor (HEMT). These power transistor devices use a restrictedcarrier channel layer that reduces current flow in a drift region of thedevice. The restricted carrier channel is formed by selectively removingone or more portions of the donor-supply layer and filling the spacewith passivation material so that a carrier channel layer does not formdirectly under the removed portions. These one or more portions may bethrough-holes or have not enough donor-supply layer material remainingto form a carrier channel layer. The passivation material has a lowerdielectric constant than the material it replaces. In most cases, thepassivation material has a dielectric constant less than about 9. Thisstructure has a higher breakdown voltage threshold than a comparativestructure without the restricted carrier channel layer.

Referring to FIG. 1A, the power transistor device 100 a is formed over asilicon substrate 101. A number of layers are grown over a substrate 101using an epitaxial process. The layers include an optional nucleationlayer of aluminum nitride layer (not shown), an optional buffer layer(not shown), and a bulk gallium nitride layer 103, which may be grownover the buffer layer or directly on a substrate. The bulk galliumnitride (GaN) layer 103 is a channel layer for the HEMT 100 a.

FIG. 1A shows an active layer 105 on top of the bulk GaN layer 103. Theactive layer 105, also referred to as donor-supply layer, is grown onthe channel layer 103. An interface is defined between the channel layer103 and the donor-supply layer 105. A carrier channel 107 oftwo-dimensional electron gas (2-DEG) is located at the interface. In atleast one embodiment, the donor-supply 105 is an aluminum galliumnitride (AlGaN) layer (also referred to as the AlGaN layer 105). TheAlGaN layer 105 has a formula of Al_(x)Ga_((1-x))N, wherein x variesbetween about between about 10% to 100%. It has a thickness in a rangefrom about 5 nanometers to about 50 nanometers. In other embodiments,the donor-supply layer 105 may include an AlGaAs layer, or AlInP layer.

A band gap discontinuity exists between the AlGaN layer 105 and the GaNlayer 103. The electrons from a piezoelectric effect in the AlGaN layer105 drop into the GaN layer 103, creating a very thin layer 107 ofhighly mobile conducting electrons in the GaN layer 103. This thin layer107 is referred to as a two-dimensional electron gas (2-DEG), forming acarrier channel (also referred to as the carrier channel 107). The thinlayer 107 of 2-DEG is located at an interface of the AlGaN layer 105 andthe GaN layer 103. Thus, the carrier channel has high electron mobilitybecause the GaN layer 103 is undoped or unintentionally doped, and theelectrons can move freely without collision or substantially reducedcollision with the impurities.

A source feature 111 and a drain feature 113 are disposed on the AlGaNlayer 105 and configured to electrically connect to the carrier channel107. Each of the source feature and the drain feature comprises acorresponding intermetallic compound. The intermetallic compound is maybe embedded in the AlGaN layer 105 and may be further embedded in a topportion of the GaN layer 103. In one example, the intermetallic compoundcomprises Al, Ti, or Cu. In another example, the intermetallic compoundcomprises AlN, TiN, Al₃Ti or AlTiN.

The semiconductor structure 100 a also includes a gate structure 117disposed on the AlGaN layer 105 between the source 111 and drain 113features. The gate 117 includes a conductive material layer whichfunctions as the gate electrode configured for voltage bias andelectrical coupling with the carrier channel 107. In various examples,the conductive material layer may include a refractory metal or itscompounds, e.g., tungsten (W), titanium nitride (TiN) and tantalum (Ta).Other commonly used metals in the conductive material layer includenickel (Ni) and gold (Au). The gate structure may include one or manylayers.

According to various embodiments of the present disclosure, apassivation material 109 covers a drift region of the donor-supply layer105 between the gate structure 117 and drain 113. Passivation material109 includes a first portion 109 a, also referred to as buried portion109 a, and a second portion 109 b, also referred to as a top portion 10b. The buried portion 109 a fills one or more through-holes or nearthrough-holes in the donor-supply layer 105. In FIG. 1A, the buriedportion 109 a contacts the underlying bulk GaN layer 103. The topportion 105 of the passivation material 109 b has a relatively constantheight, which also may be referred to as a top thickness. The carrierchannel 107 has patches of non-conduction where the buried portion 109 aof the passivation material contacts the bulk GaN layer 103. As result,the total current for the power transistor device during operation isreduced and resistance between the drain and source is increased, butthe breakdown voltage threshold increases.

The passivation material 109 may be silicon oxide, silicon nitride,silicon oxynitride, carbon doped silicon oxide, carbon doped siliconnitride, carbon doped silicon oxynitride, zinc oxide, zirconium oxide,hafnium oxide or titanium oxide. The dielectric layer is deposited usinga process such as chemical vapor deposition (CVD). A dielectric material119 over the passivation material 109 covers the source 111 and drain113.

In other embodiments, such as power transistor device 100 b shown inFIG. 1B, the buried portion 109 a does not contact the underlying bulkGaN layer 103. Only a very thin layer of the donor-supply layer remainsover the bulk GaN layer 103, shown as 105 b in FIG. 1B. In theseembodiments, the very thin layer 105 b of the donor-supply layer 105 hasa thickness of about 3 nm or less, such that the buried portion 109 a issaid to be almost contacting the underlying bulk GaN layer 103. Thisvery thin layer 105 b is not enough to form a carrier channel layer 107with the underlying bulk GaN layer 103, and the carrier channel layer107 is also discontinuous as shown in FIG. 1B.

In yet other embodiments, the power transistor device may includeadditional features that modulate the surface electric field at the gateedge. As shown in FIG. 1C for device 100 c, an additional passivationmaterial 121 is deposited beneath the gate 117 edge. Used in conjunctionwith buried passivation material 109 a, the maximum surface electricfield which usually occurs at the gate edge is also reduced. Anotherexample of gate edge surface electric field modulation is the use offield plates. FIG. 1D shows an example power device 100 d having a fieldplate 123 formed over the gate 117 edge. The field plate is formed ofmetal and its proximity to the gate edge reduces the maximum surfaceelectric field at the gate edge.

While FIGS. 1A to 1D show one buried portion 109 a, the passivationmaterial 109 may include one or many buried portions 109 a as shown inFIG. 2A. FIGS. 2A and 2B are example top views of various buriedportions 209 in accordance with various embodiments of the presentdisclosure. The top views shows the HEMT 100 a of FIG. 1A cut from line110. Line 110 separates the passivation material 109 into top portionsand buried portions.

FIG. 2A shows a total of five buried portions 209 in the drift region.Five buried portions 209 are dispersed between the gate structure 217and drain 213, although fewer or more buried portions may be used. Theburied portions 209 create non-conductive patches in the conductivesurface of the carrier channel layer below. The surface area of theburied portion 209 is more than about 10% and is not greater than about50% of the total surface area between the gate 217 and drain 213. Whilelarger surface area of the buried portion 209 can be created and used,the on-state current may be sufficiently reduced such that any breakdownvoltage gain is not worth the loss in transistor performance. Anotherway to review this limitation is the effective width of the carrierchannel layer. Because patches of non-conduction are inserted, theeffective width is reduced by the total width of the buried portions209. In other words, a minimum width of the carrier channel layer isabout 50% to about 90% of the width of the bulk GaN layer.

FIG. 2B shows a single buried portion 209 in the center of the driftregion as another example. This design is simpler to implement and maybe easier to fill depending on the overall geometry of the device. Aratio of a buried portion and adjacent drift region that does notinclude buried portion may be may be about 1:1 to about 1:9.

The various buried portions shown in FIGS. 2A and 2B are merelyexamples. The buried portions may be polygons, such as quadrilateralsshown in figures. The buried portions may have more than four sides ormay be circular or irregular. For geometries that have a varying carrierchannel width, the minimum width rule of having about 50% to about 90%of the width of the bulk GaN layer applies. In other words, the buriedportions 209 of the passivation material can only reduce the carrierchannel width by about 10% to about 50%.

FIGS. 3 and 4 are process flow charts that illustrate various operationsof a method to form the HEMTs of the present disclosure. FIGS. 3 and 4are discussed in association with FIGS. 5A to 5E that shows a partiallyfabricated HEMT in various stages of manufacturing. FIG. 3 illustrates aprocess flow 300 that describes operations to make a HEMT in accordancewith various embodiments of the present disclosure. In the firstoperation 301, a silicon substrate is provided. FIGS. 5A to 5E show thesilicon substrate 501. Note that the various elements in the figures arenot drawn to scale. While HEMTs are often manufactured on a substrateother than silicon substrate, such as sapphire and silicon carbide, themethods and structures disclosed herein are specific silicon fabricationprocesses and silicon-based structures.

Referring to FIG. 3, in operation 303, a bulk layer of undoped galliumnitride (UID GaN) is epitaxially grown over the silicon substrate. TheUID GaN layer is shown in FIGS. 5A to 5E as layer 509 over the siliconsubstrate 501. The bulk layer of undoped gallium nitride 509 isepitaxially grown over the silicon substrate 501, which may includeintervening layers such as a nucleation layer and/or a buffer layer. Thebulk layer of gallium nitride 509 does not include any dopant, but mayinclude contaminants or impurities that are incorporated in the filmunintentionally. The bulk layer of gallium nitride may be referred to asunintentionally doped gallium nitride (UID GaN) layer. The UID galliumnitride may be about 0.5 microns to about 1 micron thick. The bulk GaNlayer is grown under high temperature conditions. The process may bemetal organic CVD (MOCVD), metal organic vapor phase epitaxy (MOVPE),plasma enhanced CVD (PECVD), remote plasma enhanced CVD (RP-CVD),molecular beam epitaxy (MBE), hydride vapor phase epitaxy (HVPE),chloride vapor-phase epitaxy (Cl-VPE), and/or liquid phase epitaxy(LPE). Using metal organic vapor phase epitaxy (MOVPE) involves agallium-containing precursor and nitrogen-containing precursor. Thegallium-containing precursor includes trimethylgallium (TMG),triethylgallium (TEG), or other suitable chemical. Thenitrogen-containing precursor includes ammonia (NH.sub.3),trimethylalaluminum (TMA), phenyl hydrazine, or other suitable chemical.

Referring to FIG. 3, in operation 305, a donor-supply layer is grownover the UID GaN layer. The donor-supply layer is shown in FIGS. 5A to5E as layer 511 over the UID GaN layer 509. The donor-supply layer 511can be epitaxially grown on the GaN layer 509 by MOVPE using analuminum-containing precursor, a gallium-containing precursor, and anitrogen-containing precursor. The aluminum-containing precursorincludes TMA, TEA, or other suitable chemical. The gallium-containingprecursor includes TMG, TEG, or other suitable chemical. Thenitrogen-containing precursor includes ammonia, tertiarybutylamine(TBAm), phenyl hydrazine, or other suitable chemical.

The band gap discontinuity between the donor-supply layer 511 and theUID GaN layer 509 forms a carrier channel 513 at the interface betweenthe two layers 511 and 509. The carrier channel 513 is al called atwo-dimensional electron gas (2-DEG), where electrons have high mobilitywhen the transistor is on.

Referring to FIG. 3, in operation 307, a portion of the donor-supplylayer 511 is etched to a level at a top surface of the UID GaN layer509, slightly below, or slightly above—leaving a small portion of thedonor-supply layer 511 remaining. As discussed herein in associationwith FIGS. 1A and 1B, various embodiments of the present disclosureinclude devices that have patches of non-conduction in the carrierchannel layer by removing sufficient amounts of the donor-supply layer511 such that the 2-DEG carrier channel 513 does not form. As shown inFIG. 5A, the carrier channel layer 513 is discontinuous at the crosssection shown because all of the donor-supply layer 511 in an opening503 is removed.

A photoresist layer may be first deposited and patterned to protectparts of the donor-supply layer that are not to be etched. The etchpattern may be one of the various configurations from FIGS. 2A and 2B,or others as discussed or designed by one skilled in the art accordingto the inventive concepts in this disclosure. Once a sufficient amountof the donor-supply layer 511 is removed, additional etching does notchange the electrical properties of the final device. Thus, a fast etchprocess with small amount of over etching may be used. While othermethods and structures using buried oxide layers involve careful etchingto not over etch the donor-supply layer, the methods in accordance withvarious embodiments of the present disclosure do not require suchcareful etching. As result, one of the advantages of various embodimentsof the present disclosure is a larger process window for the method ofmaking the power device. In some embodiments, a fluorine-based etchantor a chlorine-based etchant is used in a plasma to etch the opening. Insome embodiments, the etch process may be very fast initially, but asthe bottom of the etched opening approaches the UID GaN layer 509, theprocess may be slowed down, for example, the plasma power and pressuremay be reduced in the chamber. FIG. 5A shows the partially fabricatedHEMT with an opening 503 in the donor-supply layer 511. Although onlyone opening 503 is shown, various embodiments include any number of atleast one opening as discussed in association with FIGS. 2A and 2B.

Once the openings or cavities are formed in the donor-supply layer, thephotoresist protecting parts of the donor-supply layer is removed. Inoperation 309, a passivation material layer is deposited over thedonor-supply layer and in the opening. As shown in FIG. 5B, apassivation material 505 is deposited. The passivation material 505fills the openings etched in operation 307 of FIG. 3 and completelycovers the donor-supply layer. The dielectric layer may be siliconoxide, silicon nitride, silicon oxynitride, carbon doped silicon oxide,carbon doped silicon nitride, carbon doped silicon oxynitride, zincoxide, zirconium oxide, hafnium oxide or titanium oxide. The dielectriclayer is deposited using a process such as chemical vapor deposition(CVD).

The passivation material 505 is deposited in one operation and forms asingle material layer. A first portion 505 a of the passivation material505 is located in the etched portions of the donor-supply layer andfills the opening 503. A bottom surface of the first portion 505 a maycontact the UID GaN layer 509 or a small remaining portion of thedonor-supply layer 511 (for example, about or less than 3 nm). A secondportion 505 b of the passivation material 505 is located above thedonor-supply layer 511. A top surface of the second portion 505 b isrelatively flat even over the opening because the etched openings 503have low aspect-ratios.

In operation 311, a source and a drain is formed on the donor-supplylayer. FIG. 5C shows the source 515 and the drain 517 on thedonor-supply layer 511. In some embodiments, the source 515 and thedrain 517 are intermetallic compounds or alloys. The intermetalliccompound may be formed by constructing a patterned metal layer over thedonor supply layer 511 or fully or partially in a cavity of thedonor-supply layer 511. The patterned metal layer may then be annealedsuch that the metal layer, the donor-supply layer 511 and optionally theGaN layer 509 react to form the intermetallic compound. Because of theanneal, the metal elements in the intermetallic compound may diffuseinto the donor-supply layer 511 and the GaN layer 509. The intermetalliccompound contacts the carrier channel 513 located at the interface ofthe donor-supply layer 511 and the GaN layer 509. The intermetalliccompound may improve electrical connection and form ohmic contactsbetween the source/drain features and the carrier channel 513. In oneexample, the intermetallic compound is formed in the cavity of thedonor-supply layer 511 as well as a portion of the donor-supply layeraround the cavity such that the intermetallic compound follows thecontour of the donor-supply layer 511 and has a non-flat top surface. Inanother example, intermetallic compound overlies a portion of the AlGaNlayer 511.

FIG. 4 further separates the operation 311 into detailed sub-operations.In operation 421, a source opening and a drain opening are etched in thepassivation material layer. A photoresist or etch mask is firstdeposited to protect portions of the passivation layer not to be etched.A dry etch process may be used to form the source opening and drainopening.

In operation 423, a number of metal layers are deposited over thedonor-supply layer and in openings etched in the donor-supply layer forthe source and the drain. The metal layers may include titanium,aluminum, and copper. In one example, the metal layers are a titanium,aluminum, and titanium stack. The metal layers are deposited using aphysical vapor deposition (PVD) process. A plasma is formed in a chamberproximate to a target containing the metal to be deposited. Ions in theplasma bombard the target causing metal particles to sputter off. Thesputtered metal particles deposit onto the substrate forming a metallayer. The metal layers conform to the surface topography of thedonor-supply layer and the openings.

In operation 425, a source region and a drain region of the metal layerare covered. Usually, a photolithographic process is used to cover themetal layers to be protected. A photoresist is deposited and exposed toa light pattern that modifies the property of the photoresist. Dependingon the type of photoresist, exposed portions or unexposed portions areremoved by developing the photoresist, leaving only some photoresist onthe metal layers. The covering is used as a mask in subsequentprocessing. Other materials such as a patterned hardmask may be usedalso. The hardmask would be deposited first, patterned, and etched todefine the source and drain.

In operation 427, the metal layers are etched. The mask or photoresistfrom operation 425 is an etch mask that protects the source and drainfrom being removed. Dry etch using plasma removes the metal layers thatare not protected, down to the donor-supply layer. Suitable plasma etchtechniques include a reactive ion etch (RIE) and an inductively coupledplasma (ICP) etch. Depending on the etch selectivity of the metal overthe underlying passivation material layer, the metal layer may beslightly over-etched to ensure complete removal of the metal layers overthe passivation material layer 505.

In operation 429, the covering over the source and the drain is removed.If the covering is photoresist, then the removal process is ashing andstripping. If the covering is a hardmask, then the removal process mayinvolve etching with different plasma etchants from that of operation427. At this point the partially fabricated structure resembles thestructure of FIG. 5C. Thereafter in operation 429, the source and thedrain may be annealed in a rapid thermal anneal (RTA) process. Annealingcan create an intermetallic compound or an alloy. In one embodiment, analloy is created in an inert environment at about 800 to 900 degreesCelsius. In another embodiment, an intermetallic compound containingnitrogen is created in a nitrogen environment. Other annealingenvironments include an oxygen environment for creating some conductiveoxides and an ambient environment. As discussed, annealing the sourceand drain may also allow interaction of the metal layers with underlyingmaterial from the donor-supply layer. The result may be a source anddrain that includes material from the underlying layers. In effect, alarger source and/or drain than the metal layers are created becausesome metal diffuses into the donor-supply layer and some aluminum,gallium, and nitrogen diffuses into the source and the drain.

As shown in FIG. 5D, after forming the source 515 and drain 517, adielectric layer 507 may be deposited over the source, drain, and thepassivation material. However, the dielectric layer 507 is optional andin some cases may be of the same material as the passivation materiallayer 505.

Referring to FIG. 3, in operation 313, a gate structure is formedbetween the source and the drain. Similar to the source/drain formationprocess, the gate structure is formed in a number of steps. According tosome embodiments, the gate structure may be defined first by aphotolithography process through the passivation material 505 anddielectric 507. The gate material is then deposited in a CVD process ora metal CVD process over the donor-supply layer. As discussed, the gatematerial may include one or more of tungsten (W), titanium nitride (TiN)and tantalum (Ta). Excess gate material is then etched away to achievethe structure of FIG. 5E. The structure of FIG. 5E includes a gatestructure 519 between a source 515 and a drain 517 over the donor-supplylayer 511.

Referring to FIG. 3, in operation 315, contacts are deposited over thegate, the source, and the drain. A photoresist is deposited andpatterned to expose the source, the drain, and the gate structure. Thedielectric layer over the source, the drain, and the gate structure areremoved with an etch process. After the basic HEMT structure is formed,subsequent operations include depositing contacts and first layer ofmetal interconnect along with intermetallic dielectric (IMD) materialusing suitable semiconductor processes and designs.

FIGS. 6A and 6B are representations of surface electric field as afunction of distance on the HEMT structures in accordance with variousembodiments of the present disclosure. Electric field is usuallyrepresented in volts per centimeter in a logarithmic scale and isplotted against a distance along a line across the HEMT. The surfaceelectric field in FIGS. 6A and 6B are shown against the device elementswith dotted lines. Line 601 of FIG. 6A represents a comparative HEMTwithout the buried portions as disclosed herein. Point 603 representsthe source edge closest to the gate. Point 605 corresponds to the gatestructure edge closest to the drain. Point 607 corresponds to the drainedge closest to the gate. FIG. 6A shows that the surface electric fieldis highest at the gate edge closest to the drain.

FIG. 6B represents a HEMT having a buried portion in accordance withvarious embodiments of the present disclosure. Line 611 is the surfaceelectric field. Point 613 represents the source edge closest to thegate. Point 615 corresponds to the gate structure edge closest to thedrain. Point 617 represents the edge of the buried portion closest tothe gate. Point 619 represents the edge of the buried portion closest tothe drain. Point 621 corresponds to the drain edge closest to the gate.Comparing FIGS. 6A and 6B, the peak surface electric field are both atthe gate edge closest to the drain, points 605 and 615 respectively. Theintensity of the peak surface electric fields are about the same, as theburied portions do not affect the surface electric field at the gateedge. In other words, having the buried portions as disclosed hereindoes not change the breakdown voltage threshold as is affected by thegate edge surface electric field. However, the buried portions affectthe surface electric field in the drift region between the gate and thedrain. As shown in FIG. 6B, the surface electric field across the HEMTis increased over the portions of the drift region where the buriedportions lie. Thus, the overall electric field at the surface isincreased for the device without affecting the peak electric field.

The embodiments of the present disclosure may have other variations. Forexample, the buried portions may include more than one material, such asa layer of nickel oxide over a layer of gallium nitride. Certainembodiments of the present disclosure have several advantageousfeatures. The use of various doping species allows fine-tuning of theburied portions, and hence the breakdown voltage, while minimizingadverse effects to other electrical properties, such as maximum forwardcurrent or leakage current.

An aspect of this description is related to a transistor. The transistorcomprises a first layer over a substrate. The transistor also comprisesa second layer over the first layer. The transistor further comprises acarrier channel layer at an interface of the first layer and the secondlayer. The transistor additionally comprises a gate structure, a drain,and a source over the second layer. The transistor also comprises apassivation material in the second layer between an edge of the gatestructure and an edge of the drain in a top-side view. The carrierchannel layer has a smaller surface area than the first layer betweenthe edge of the gate structure and the edge of the drain in the top-sideview.

Another aspect of this description is related to a transistor. Thetransistor comprises a first layer over a substrate. The transistor alsocomprises a second layer over the first layer. The transistor furthercomprises a carrier channel layer at an interface of the first layer andthe second layer. The transistor additionally comprises a gatestructure, a drain, and a source over the second layer. A drift regionis between an edge of the gate structure and an edge of the drain in atop-side view. The transistor also comprises non-conductive patches inthe drift region. An entirety of at least one of the non-conductivepatches is in the drain in the drift region in the top side view. Thecarrier channel layer is discontinuous in the drift region.

A further aspect of this description is related to a method comprisingepitaxially growing a first layer over a silicon substrate. The methodalso comprises epitaxially growing a second layer over the first layer.The method further comprises forming a gate structure, a drain, and asource over the second layer. The method additionally comprises removingone or more portions of the second layer to form one or morethrough-holes exposing the first layer between an edge of the gatestructure and an edge of the drain in a top-side view. The method alsocomprises filling the one or more through holes with a non-conductivematerial, forming one or more discontinuities in a carrier channel layerat an interface between the first layer and the second layer.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A device comprising: a first layer; a secondlayer over the first layer; a carrier channel layer at an interface ofthe first layer and the second layer; a gate structure disposed over thesecond layer; a first source/drain feature on a first side of the gatestructure and a second source/drain feature on a second side of the gatestructure, the second side being opposite the first side; a passivationmaterial having a first portion extending from the first source/drainfeature to the first side of the gate structure and a second portionextending from the second side of the gate structure to the secondsource/drain feature, wherein the passivation material does not extenddirectly under the gate structure such that no portion of thepassivation material is positioned directly under the gate structure;and an another passivation layer disposed within the second layerbetween the second side of the gate structure and the secondsource/drain feature, wherein the second portion of the passivationmaterial is disposed over the another passivation layer.
 2. The deviceof claim 1, wherein the first layer includes a first semiconductormaterial and the second layer includes a second semiconductor materialthat is different than the first semiconductor material.
 3. The deviceof claim 1, wherein the passivation material extends into the firstlayer.
 4. The device of claim 1, wherein the passivation materialextends into the second layer without extending to the first layer. 5.The device of claim 1, wherein the first layer includes gallium nitrideand the second layer includes aluminum gallium nitride.
 6. The device ofclaim 1, further comprising a semiconductor substrate, and wherein thefirst and second layers are disposed over the semiconductor substrate.7. The device of claim 1, wherein the passivation material extends intothe first layer, and wherein the another passivation layer does notextend into the first layer.
 8. The device of claim 7, wherein thesecond portion of the passivation material extends into the first layer.9. A device comprising: a first semiconductor layer; a secondsemiconductor layer disposed over the first semiconductor layer; acarrier channel at an interface of the first semiconductor layer and thesecond semiconductor layer; a gate structure disposed over the secondsemiconductor layer; a source/drain feature disposed over the secondsemiconductor layer; a drift region extending from an edge of the gatestructure to an edge of the source/drain feature; and a non-conductivepatch disposed in the drift region without extending directly under thegate structure, wherein the non-conductive patch includes a firstportion having a first width and a second portion having a second widththat is different than the first portion.
 10. The device of claim 9,wherein the first semiconductor layer is formed of a differentsemiconductor material than the second semiconductor layer.
 11. Thedevice of claim 9, wherein the non-conductive patch includes a pluralityof non-conductive patches.
 12. The device of claim 9, wherein thenon-conductive patch extends to at least the first semiconductor layer.13. The device of claim 9, wherein the non-conductive patch extends intothe first semiconductor layer without extending to the secondsemiconductor layer.
 14. The device of claim 9, wherein the first widthis wider than the second width and the first portion is disposed overthe second portion.
 15. A device comprising: a first layer; a secondlayer disposed over the first layer; a carrier channel layer at aninterface of the first layer and the second layer; a gate structuredisposed over the second layer; a source/drain feature disposed on thesecond layer; a non-conductive patch extending from a side of the gatestructure to the source/drain feature and into the first layer, whereinthe non-conductive patch includes a first portion having a first widthand a second portion having a second width that is less than the firstportion, wherein the first portion is disposed over the second portion;and a field plate disposed on the gate structure.
 16. The device ofclaim 15, further comprising a dielectric layer disposed over thenon-conductive patch.
 17. The device of claim 16, wherein the dielectriclayer extends from the side of the gate structure to the source/drainfeature.
 18. The device of claim 16, wherein the field plate extendsover the dielectric layer such that a portion of the field platephysically contacts the dielectric layer.
 19. The device of claim 15,wherein the first layer includes a first semiconductor material and thesecond layer includes a second semiconductor material that is differentthan the first semiconductor material.
 20. The device of claim 15,wherein the second portion of the non-conductive patch extends into thefirst layer.